HIPERCOAT Informe resumido

TBC/TGO interface interactions

Rare earth zirconates are of interest in thermal barrier coatings (TBCs) because they combine low thermal conductivity with improved morphological stability relative to the standard material based on yttria stabilized zirconia (7YSZ).

Based on the experimental information on the alumina-yttria-zirconia system available at the time and the similarities in phase constitution of the REO-zirconia and REO-alumina binaries, it was postulated at the start of this project that there would be a limit to the thermochemical compatibility of RE doped zirconias with the thermally grown alumina, which provides oxidation protection to the underlying super-alloy in a gas turbine component. A corollary was that the RE zirconate compositions would probably be thermochemically incompatible with alumina in all cases.

Studies under this program demonstrated that interphases do form when RE zirconates are placed in contact with alumina at high temperature. This was ascertained for Gd and La, whose zirconates are of significant interest in the current TBC literature. Detailed studies on Gd zirconate deposited by EB-PVD on alumina as well as alumina-forming metallic substrates revealed that the reaction is sufficiently rapid at 1200°C (~1µm after 100h) to jeopardize the integrity of the coating. The process involves concurrent counter-diffusion of Gd and Al and yields a Gd aluminates interphase next to the alumina, overlaid by a layer of cubic zirconia whose Gd content increases with distance away from the interface. Formation of the aluminate layer is accompanied by evolution of pores at its interface with alumina, degrading adherence and compromising the integrity of the coating.

It was also shown that the reaction is less severe but still significant at 1100°C (~100nm after 100h), suggesting that the maximum allowable temperature at the TGO/TBC interface should be substantially lower to avoid interphase formation over the required life of the coating system. Additional studies, still in progress, revealed that the kinetics is strongly dependent on the actual Gd content of the zirconate, which is a solid-solution pyrochlore phase. Because the thermal conductivity does not vary significantly over the same composition range, the results suggest that substoichiometric zirconate phases may be preferred for actual applications to reduce the risk of interface interactions. Compatible inter-layers are also a possible solution, as detailed in a separate result.